Add Build Event Service (BES) support

The [Build Event Protocol](https://bazel.build/remote/bep) (BEP) allows third-party programs to gain insight into a Bazel invocation. It includes information about build and test results, build progress, the build configuration and much more. The [Build Event Service](https://bazel.build/remote/bep#build-event-service) (BES) Protocol is a generic gRPC service for publishing BEP events.

Cloud-based RBE systems intended for use with Bazel builds often use BES reporting to track progress and resource utilization associated with a particular build. Lack of BES support can be a blocker for reclient use with these systems.

No repository files, tests, or entry points are named. Start by reviewing the linked Build Event Protocol and Build Event Service documentation, then map the required integration points in reclient. Done means reclient can publish BEP events to a BES endpoint and the behavior is covered by appropriate tests.

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